Rebels in Myanmar were using various 3d printed guns just after the military coup (famously the FGC-9), which is like a PDW form factor chambered in 9mm. The barrels are metal, and i think the chamber as well, but the whole fire control group i think is all printed and of course all the furniture is plastic as well.

https://www.youtube.com/watch?v=2K1qXxONls4

well, it's not because they shopped around and were like - yeah, we don't like these AK-74s and ar15s, let's just use FGC-9 instead.